Optical Absorption Spectra and Excitons of Dye-Substrate Interfaces: Catechol on TiO$_2$(110)

TL;DR

This study uses advanced computational methods to analyze the optical absorption and excitonic properties of catechol on TiO2 interfaces, providing insights into charge transfer mechanisms relevant for improving dye-sensitized solar cells.

Contribution

It presents a detailed first-principles calculation of interfacial excitons and absorption spectra for catechol on TiO2, highlighting the effects of deprotonation and coverage on photovoltaic efficiency.

Findings

Deprotonation shifts absorption onset to lower energies.

Bright excitons are intense charge transfer transitions with spatially separated charge carriers.

Results semi-quantitatively agree with experimental spectra.

Abstract

Optimizing the photovoltaic efficiency of dye-sensitized solar cells (DSSC) based on staggered gap heterojunctions requires a detailed understanding of sub-band gap transitions in the visible from the dye directly to the substrate's conduction band (CB) (type-II DSSCs). Here, we calculate the optical absorption spectra and spatial distribution of bright excitons in the visible region for a prototypical DSSC, catechol on rutile TiO$_2$(110), as a function of coverage and deprotonation of the OH anchoring groups. This is accomplished by solving the Bethe-Salpeter equation (BSE) based on hybrid range-separated exchange and correlation functional (HSE06) density functional theory (DFT) calculations. Such a treatment is necessary to accurately describe the interfacial level alignment and the weakly bound charge transfer transitions that are the dominant absorption mechanism in type-II DSSCs. Our HSE06 BSE spectra agree semi-quantitatively with spectra measured for catechol on anatase TiO$_2$ nanoparticles. Our results suggest deprotonation of catechol's OH anchoring groups, while being nearly isoenergetic at high coverages, shifts the onset of the absorption spectra to lower energies, with a concomitant increase in photovoltaic efficiency. Further, the most relevant bright excitons in the visible region are rather intense charge transfer transitions with the electron and hole spatially separated in both the [110] and [001] directions. Such detailed information on the absorption spectra and excitons is only accessible via periodic models of the combined dye-substrate interface.